A common motor system for motor vehicles is designed to boost a battery voltage by a specific booster, and to apply the boosted voltage to an inverter, thus achieving higher output power with low loss and improving the ability of the motor system to respond to demands higher RPM.
In order to omit the specific booster, U.S. Pat. No. 6,320,775 corresponding to Japanese Patent Application Publication NO. H10-337047 discloses a booster system. The booster system is designed to apply a battery voltage to a neutral point of star-connected multiphase stator windings of a brushless motor to thereby serve each of the multiphase stator windings as a reactor. The configuration of the booster system allows increase in magnetic energy stored in each of the multiphase stator windings, thus boosting a voltage to be applied to an inverter for driving the multiphase stator windings. Such a boosting system disclosed in U.S. Pat. No. 6,320,775 will be referred to as “neutral-point powering booster system” hereinafter.
However, it is known in the art that the neutral-point powering booster system disclosed in U.S. Pat. No. 6,320,775 increases third harmonic wave components in a direct current supplied from a power supply source due to variations in the potential of the neutral point; this third harmonic wave components are superimposed on a fundamental wave component of an armature current (stator current) per phase.
FIG. 5 schematically illustrates waveforms of the fundamental wave component of the U-phase stator current Iu1 and the direct current Idc1 supplied from the power supply source of the neutral-point powering booster system; this direct current Idc1 contains the third harmonic wave components.
Specifically, FIG. 5 clearly shows that the direct current Idc1 periodically varies due to the third harmonic wave components.
In order to reduce the third harmonic wave components in the direct current, Japanese Patent Application Publication No. 2002-218793 discloses a system including a pair of star-connected multiphase stator windings and a pair of multiphase inverters for driving the respective star-connected multiphase windings.
The system is designed such that a positive electrode of a power supply source is connected with a neutral point of one of the pair of multiphase stator windings and a negative electrode of the power supply source is connected with that of the other thereof. Such a system disclosed in Japanese Patent Application Publication No. 2002-218793 will be referred to as “inter neutral-point powering system” hereinafter.
The inter neutral-point powering system is designed to use a difference in potential between the neutral point of one of the pair of multiphase stator windings and that of the other thereof. For this reason, the inter neutral-point powering system may be difficult to achieve commonality in potential between DC terminals of one of the pair of multiphase inverters and those of the other thereof.
This results in that a low-side DC potential of the low-side DC terminal of one of the pair of the multiphase inverters has a negative potential lower than a ground potential. This may make difficult that such an inter neutral-point powering system is applied to in-vehicle electric systems with vehicle body ground (body earth), such as in-vehicle electric systems with batteries for accessories.